LU-1581 utils: mkfs warn about missing --index

LU-1581 utils: mkfs warn about missing --index

Using the --index option to mkfs.lustre is standard practice, and is
documented as required in the Lustre manual.  Add a warning for mkfs
usage in the 2.3 release, so that users with formatting scripts/tools
can be notified of this change in advance of the hard requirement in
Lustre 2.4.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I1f481a334163af1033da5296f9e71fb3b4cee07e
Reviewed-on: http://review.whamcloud.com/3703
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Li Wei <liwei@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>